1. Field of the Invention
The present invention relates to a capacitor discharge type ignition system for an internal combustion engine, and more particularly to an overrun prevention ignition system having an ignition angle retardation circuit.
2. Description of the Prior Art
An ignition system having an ignition angle retardation circuit to prevent the overrun of an engine is known, for example by Japanese Published Unexamined Patent Application No. 55-5451. In this type of ignition system, an auxiliary capacitor is charged in parallel with the charging of a main capacitor (or a charging capacitor) and the charge stored is discharged through a discharging circuit connected to a base of a switching transistor which is connected in parallel with a gate-cathode circuit of a main thyristor (or ignition controlling thyristor). In this manner the switching transistor is rendered conductive for a predetermined time period. As the rotation of the engine increases to shorten a period of a reverse electromotive force of a generator coil which serves as an ignition signal, the ignition signal partially overlaps the predetermined (constant) conduction period of the switching transistor. Thus, the ignition timing is delayed by the overlapping period. A component (opposite phase electromotive force) of the elecytromotive force generated in the generator coil of a magneto, which is opposite to the electromotive force component to charge the main capacitor is used as the ignition signal to trigger the main thyristor. In order to attain a sufficient amount of retardation angle, it is necessary to increase the overlapping period of the ignition signal and the constant conduction period of the switching transistor. As a result, it is necessary to generate the ignition signal having a long effective duration (a width during which the signal exceeds a trigger level). This requirement may be met to some extent by the selection of the dimensions of a magnet rotor and a core of the magneto but there is a limitation. It is also possible to wind on the magnet rotor of the magneto a special signal coil which can generate an ignition signal of longer duration but this results in an increase in the dimensions of the magneto and the signal coil.